The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the line pressure in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of the throttle valve of the engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of the engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (upshift). At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed which is determined by the line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio.
In such a system, it is preferable to shift the spool of the line pressure control valve by a control oil supplied to an end of the spool and to control the line pressure by controlling the amount of the control oil. The control of the amount of the control oil is carried out by controlling the draining of the control oil by a solenoid operated valve which is intermittently operated by pulses. Thus, by controlling the duty ratio of the pulses, the line pressure is controlled to its necessary value.
However, a small variation of the pressure of the control oil causes a large variation of the line pressure. For example, a variation of 4 Kg/cm.sup.2 in the control oil pressure causes a variation of 20 Kg/cm.sup.2 in the line pressure. Accordingly, if the viscosity of the control oil changes dependent on the change of temperature, the line pressure greatly changes. Therefore, the system must be provided with temperature compensating means, which renders the system complicated in construction.